Conventionally, the cathode structure used in an electron tube of a TV, etc. comprises, as shown in FIG. 1, a cathode sleeve 1 cylindrically shaped to have an upper heat radiation part 1a and lower supporting part 1b having different diameters, the cathode sleeve being made of Ni-Cr alloy, a nickel sleeve cap 2 containing minute amount of reducing metal for closing the upper end of the upper heat radiation part 1a, an electron emissive substance 3 applied to the outer surface of the sleeve cap 2, a cathode sleeve holder 4 for holding the lower supporting part 1b of the cathode sleeve 1 and containing a heater 5 for heating the cathode.
In this cathode structure, the cathode sleeve 1 is blackened by oxidizing the Cr contained in the cathode sleeve in order to improve the heat radiation, thereby reducing the time during which the heater 5 is supplied with a voltage so as to cause a picture signal to appear on the screen of the electron tube. This time is hereinafter referred to as picture appearing time.
In other words, the picture appearing time is closely related to the rate of the heat radiation of the cathode sleeve 1, which rate is considerably increased by oxidizing the Cr contained in the cathode sleeve 1 in a wet hydrogen ambient at the temperature of 1100.degree. C. The rate of the heat radiation of the blackened cathode sleeve 1 is about four times that of the unblackened cathode sleeve, and thus the picture appearing time is reduced to about a quarter of that in the case of the unblackened cathode sleeve.
However, it is conventionally hardly possible to attach the blackened cathode sleeve to the cathode sleeve holder 4 by resistance welding, and thus the blackening of the cathode sleeve inherently involves many difficulties. As a result, the blackening of the cathode sleeve is generally omitted, thus delaying the picture appearing time.